The invention pertains to the field of anti-pop circuitry for electronic appliances including at least one loudspeaker.
It is known in such appliances that during some operations of the appliance an undesired sound, known as a pop or click sound, may be heard, coming from the loudspeaker. Five operations can trigger said pop sound. They are xe2x80x9cpower onxe2x80x9d of the circuit, or going from xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d to xe2x80x9cstandbyxe2x80x9d, or for instance in a television set xe2x80x9cchanging channelxe2x80x9d, or xe2x80x9cmuting the soundxe2x80x9d, or xe2x80x9cswitching offxe2x80x9d the apparatus. Such a pop sound is caused in each of those five cases by a spurious signal which is created and introduced, when said operation are performed, at the input or output of an amplifier driving the loudspeaker. In each of said operation a power supply of the loudspeaker amplifier applies to said amplifier voltages that are changing quickly causing transient currents to flow in circuitry in an uncontrolled manner. In addition to possible damages to the loudspeaker such a pop sound is not agreeable to hear. It is the reason why, circuitry have been already designed in order to prevent any spurious signal reaching the amplifier input or output while any one of said five operations is performed. The purpose of said circuit is to control the level of a biasing signal of the amplifier and/or the level of the output, in such a way that such levels are set on or off with a steady and slow enough variation that will cause no rapid movement, of a membrane of the loudspeaker, to create an audible sound.
Examples of such attempts to reduce transient voltages at an output of an audio amplifier is shown in UK patent application nxc2x0 GB 2 096 423 (D1). EP patent application nxc2x0 EP 0562 414 A1 (D2) in the name of the applicant also shows a circuit to prevent pop sound coming from an intermediate frequency amplifier. Generally the steady and slow enough variation of the biasing or output voltage is got through an RC integrating network. The time constant of said network is great enough to prevent pop sound to occur. Such a solution is good but does not always prevent a transient arising. For instance, in D1 a prior art solution is described and discussed in relation with FIGS. 1 and 2. It is explained that due to necessary differences in time constant of RC networks connected one to the inverting input, and the other to the non inverting input of an operational amplifier of sound, a sharp transient is appearing while the amplifier is turn on. Said voltage transient is reproduced as undesired sounds by a loudspeaker connected to the amplifier (page 2 column 1 lines 42-45). To cope with this situation, it is proposed in D1 to add a control transistor 16 (see FIG. 3 of D1). Said control transistor ensures that voltage at the output of a push-pull amplifier is increasing as the charge of a capacitor. Said kind of gate transistor does not always prevent any pop from being heard. For instance in D2 it is explained that in spite of such a control transistor, a pop was still heard. To avoid said pop an anti-pop circuit has been provided at the level of an intermediate frequency amplifier. It is always difficult when conceiving a circuit to forecast all the transient currents that may appear and have an effect at the level of the sound amplifier. The present invention concerns an improvement of a circuit in which a pop sound was still heard when the apparatus was switched from on to off. As in most recent appliances, in preferred embodiments, the main functions of the apparatus are under control of a microprocessor. The micro-processor needs some time to actually detect the existing of a power interruption and to provide respective power interrupt subroutines to the different circuitry of the set. This point will be commented and explained here below.
The different voltages which are necessary for instance in a television set are produced, out of a main supply source by means of two different circuits. A first power supply circuit known in the art as switched mode power supply (SMPS) delivers two important outputs known as UB and UA supplies. UB supply is generally of 180 volts and is used for the horizontal deflection system of the cathode ray tube. UA supply is generally of about 22 volts and is used among others for the audio amplifier, for some integrated circuits processing video signal, and for producing a 5 volts supply for the microprocessor. More information about such power supply of television receivers is available for instance in two articles of VAN SCHAIK headed xe2x80x9cAn introduction to switched mode power supplies in television receiversxe2x80x9d and xe2x80x9cControl circuits for SMPS in TV receiversxe2x80x9d published pages 93 to 108 in nxc2x0 3 of vol. 34 September 1976 and pages 162 to 180 of nxc2x0 4 of said volume of December 1976 of the review xe2x80x9cElectronic applications bulletinxe2x80x9d from PHILLIPS"". Said articles have also been published in nxc2x0 135 and 136 of the British review xe2x80x9cMullard technical communicationsxe2x80x9d respectively of July and October 1977. The main reason of using the supply of the switched mode power supply to feed the microprocessor is because it is a supply which is available during stand-by time of the apparatus. The microprocessor is then still fed in order to be ready to execute orders coming for instance from a remote control of the apparatus. The second power supply is taken out of a fly-back transformer whose primary side receives a return pulse produced by an horizontal deflection coil of the cathode ray tube each time an electron beam of the tube is driven from an end of a line of the screen of the tube, to the beginning of next line. Due to this fact, voltage produced out of this second power supply is available only when the electrons beam of the tube is in scanning mode. The characteristics of all auxiliary power outputs produced out of the fly-back power supply compared to the supply produced out of the switched mode power supply are shorter discharging time and sharper roll off waveform. Said in an other way the switched mode power supply takes longer time to shut down its operation as compared to the fly-back supply. This results from the large capacitors of the switched mode power supply which are necessary to store energy to cope with sudden changes in the instant consumption and to provide the starting energy to start the apparatus when passing from stand-by to on.
An example of a known control circuit to mute the loudspeakers of an apparatus is shown in figures one and two attached to the present application. The control circuit is based on a microprocessor 1. In FIG. 1 only the features, and in particular inputs and outputs of microprocessor 1, useful for the understanding of the generation of the pop sound have been represented. Microprocessor 1 has an input 2 to detect a power interruption signal, and a reset input 3. The input 2 receives a 5 volts voltage through resistors 4 and 5. Input 2 is also connected to an interrupt circuit that will pull down voltage at input 2 when the power supply is switched off. The microprocessor detects the change of level at pin 2 and then starts interrupt sub-routines to switch off various circuit of the apparatus. The interrupt circuit comprises substantially two transistors, a first 6, a second 7, and a zener diode 8. The first and second transistors 6, 7 have each a collector 9, 10 respectively, an emitter 11, 12 respectively and a base 13, 14 respectively.
In normal operation 5 volts is supplied to pin 2 through resistors 4 and 5, as explained above. The collector 9 of first transistor 6 which is coupled to pin 2 receives also 5 volts. The base 13 of first transistor 6 is at about the same potential as the emitter 11 of said first transistor 6. Said emitter 11 is connected to earth and said base 13 is short-circuited to earth through second transistor 7 collector 10 emitter 12 path which is conducting. It results from this fact that the base emitter voltage of transistor 6 is not sufficient to make conductive the collector emitter path of transistor 6 and transistor 6 is blocked, and pin 2 is at 5 volts level. When the television set is switched off, the UA supply decreases. The voltage at the base 14 of transistor 7, which is coupled to UA voltage through zener diode 8 is dropped to almost 0 as soon as the zener diode 8 is blocked due to the decreasing of voltage UA. Transistor 7 is then blocked and no longer short-circuits base 13 of transistor 6 to earth. The voltage at the base 13 of transistor 6 which is connected to 5 volts source through resistors 15, 16 increases to a sufficient level to make conductive transistor 6. The result is that the potential of pin 2 is dropped to 0 through the collector emitter path of transistor 6. This drop of voltage is a signal for the microprocessor to start the interrupt routines, in particular an interrupt routine to mute the audio amplifier through a command circuit 40 starting from an output 17 of the microprocessor 1.
As may be understood from the process that has just been described, the detection of the power interrupt after switching off the TV set and activation of the mute command takes time. While said time is elapsing it is supposed that transient currents have already reached the level of the audio amplifier and provoked an audible pop sound.
FIG. 2 shows how the mute signal coming from output 17 of microprocessor 1 is used to drop down a voltage at mute pin 18 of an integrated audio power amplifier circuit 19. As long as a capacitor 20 having one end 21 connected to pin 18 is charged, muting is not active. End 21 of capacitor 20 is also connected to the collector 22 of a transistor 23 the base 24 of which receives the mute signal coming from mute order pin 17 of microprocessor 1. When mute signal is present, transistor 23 is conductive and capacitor 20 discharges through collector emitter path of transistor 23 and through a resistor 25. Pin 18 of integrated circuit 19 is also connected to a second discharging circuit comprising a resistor 26 and a transistor 27 in series. Base 28 of transistor 27 is coupled to UA voltage through a zener diode 29 and a diode 30 connected together in series. As long as UA is high, transistor 27 is blocked. When UA decreases because the set has been set off, transistor 27 becomes conductive, and capacitor 20 discharges through resistor 26 and emitter collector path of transistor 27, pulling down the voltage of mute pin 18 of amplifier circuit 19. To sum up in this known device each time there is a power interruption, mute pin 18 is pull down by discharging capacitor 20 through two parallel circuits, a first one, through transistor 23 controlled by a mute order coming from the microprocessor and a second one, through transistor 27 which is dependent only of the level of UA voltage. In spite of those two parallel circuits the muting is not fast enough to prevent transient currents from triggering a pop sound.
The anti-pop sound circuitry according to the invention is a fast muting circuitry that can be used with any kind of audio power amplifier included in an electronic appliance, in particular a set comprising a cathode ray tube, a power supply for the sweeping of said tube, and a muting or volume control facility. The circuit according to the invention does not cause in normal operation, any side effect to the audio power amplifier because it is not able to modify voltage present at the level of a mute or volume pin of the audio amplifier. The circuit of the invention becomes operative only when the power off has been activated. When power off has been activated, the circuit according to the invention helps in pulling down voltage present at the volume or muting control pin of the audio amplifier in a fast enough manner to prevent any transient currents that are created when off is pressed, being transformed into an amplified transient that will steer the amplifier and create an undesired pop sound.
To sum up the invention concerns an electronic appliance including a power supply fitted with two transformers, a switched mode transformer and a fly-back transformer, said appliance including also an audio amplifier and said amplifier having a mute pin, the voltage of which is dependent upon the charge of a capacitor having one end coupled to said mute pin, said capacitor being also connected to a constant voltage terminal through a first transistor, said first transistor having three electrodes, a collector an emitter and a control or base electrode and said capacitor being connected to said constant voltage terminal through the collector emitter path of said first transistor, the improvement comprising the base of said first transistor being connected to an electrode of a second transistor, said second transistor having three electrodes a collector an emitter and a base, said collector emitter path of said second transistor being coupled to a voltage source produced out of the switched mode power transformer and wherein the base of said second transistor is coupled to a voltage source produced out of the fly-back transformer, so that when switching off the electronic appliance, the second transistor becomes conductive and trigger the first transistor that becomes also conductive, causing the collector emitter path of said first transistor to discharge said capacitor and mute the audio amplifier.
It is to be noted that arrangement of power supply with a switched mode transformer and a fly -back transformer is usually found in apparatus including a cathode ray tube. In such apparatus the fly-back transformer uses pulses in synchronism with a return pulse of the electron beam of the tube, which pulses are related to horizontal deflection coil of the tube. Said synchronism avoid parasitic effects on the screen of the tube. However the invention could also be used whenever two types of power supply are provided in one apparatus, a first and a second and when voltage produced by the first supply is decreasing faster than voltage produced by the second one.